The quality of drinking water varies widely according to the amount of pollutants in the water source. Home water treatment using replaceable filter cartridges fitted in a filtered water container is a popular method to improve drinking water quality. A filter medium is found within the filter cartridge. The filter medium comprises substances and chemical compounds capable of decreasing the concentration of heavy metals such as copper, cadmium, zinc, aluminum, mercury and free chlorine. Some filter media are also capable of limiting the growth of certain microorganisms. It is known that the performance of the filter medium decreases with usage. Thus, a filter cartridge should be replaced after a known amount of water has filtered through it.
There exist mechanical and electronic mechanisms built into filtered water containers to alert the user of the need to replace the filter cartridge. All such methods and devices suffer from drawbacks.
One group of inventions merely shows the time elapsed from cartridge replacement as the method to indicate cartridge usage. Inventions such as those disclosed in U.S. Pat. Nos. 4,895,648 and 5,665,224 employ a mechanical date indicator to allow a user to record the date of insertion of a new filter cartridge. Such date indicators are intended to remind users to replace the cartridge after a given amount of time has elapsed. Such reminders are not always reliably adhered to. An electronic version of such a device was disclosed in U.S. Pat. No. 6,224,751 B1. In the device, an electronic time counter activated by a push button starts the counting. The indication of the status of cartridge usage is dependent on time elapsed from pushing the start-button. The use of the time-elapse method in calculating cartridge usage is only reliable to the extent that the estimated usage approximates the actual usage. In reality, the usage of the filter cartridge can vary tremendously from the time elapsed since its insertion. Another drawback of the time-elapse method is its reliance on the user to activate the time counting by resetting the date indicator or the electronic time counter, the omission of which renders the method useless.
Another group of inventions are mechanisms built into or around the lid of the filtered water container, which count the number of times that the lid is opened for water filling. In EP 0,861,809A1 and WO 95/29131, the lid includes a closure plate through which unfiltered water is poured into the filtered water container. The opening and closing movements of the closure plate trigger a mechanical mechanism that advances a rotatable member bearing an indicator, which purportedly indicates the state of usage of the cartridge. U.S. Pat. No. 4,986,901 disclosed a filter bottle cap fitted with a mechanism that advances a counter each time the cap is accessed for water filling.
In WO 96/13318, U.S. Pat. No. 5,900,138 and WO 00/66245, the number of times that the lid of the filtered water container is opened is also counted by mechanical, electronic or electromagnetic means. The devices disclosed in these references are based on the assumption that each time the trigger mechanisms are activated, a theoretically constant amount of water is filled into the filtered water container. However, such an assumption does not account for errors in counting such as when the trigger mechanism is inadvertently manipulated, accessed during cleaning or amounts of water that are less than the actual capacity of reservoir are used.
There also exist references that rely on flow meters to detect the amount of water that has passed through a filtration device for the indication of water filter cartridge usage. For example, U.S. Pat. No. 4,772,386 discloses a water filter cartridge attached to a housing having an impeller driven by water flow. The impeller is connected to a rotating toothed disc, which abrades and cuts a trigger wire after an estimated amount of water has flowed through the impeller. The cutting of the trigger wire shuts off the water flow. U.S. Pat. Nos. 4,681,677, 6,024,867 as well as WO 03/028848 A1 describe flow meters attached to filtration devices. In particular, U.S. Pat. No. 4,681,677 describes the use of a flow meter for monitoring water flowing into a water treatment processor. U.S. Pat. No. 6,024,86 describes the use of the movement of a ball in a water flow channel in order to detect the amount of filtered water flow. WO 03/028848 A1 describes a flow meter fitted to the lid of a filtered water container. The flow meter consists of a turbine wheel, which is rotated by the passage of unfiltered water. The trigger wire method disclosed in U.S. Pat. No. 4,772,386 is imprecise because the cutting efficiency of the toothed disc deteriorates over time and the abrasive resistance of each trigger wire can vary. As with mechanical counting mechanisms described earlier, flow meters can suffer from mechanical breakdown with wear and tear. For the flow meter described in WO 03/028848 A1, in particular, the flow meter can be inadvertently activated during movement of the lid or the filtered water container inadvertently or necessarily.
Other methods of indicating the exhaustion of a filter cartridge rely on the blockage of water flow within the cartridge. U.S. Pat. No. 3,038,610 uses a filter medium that swells upon its exhaustion to block water flow. U.S. Pat. No. 6,428,687 employs a synthetic material to block water flow. These methods, however, only indicate the duration of time that the filter medium has contacted water.
There is accordingly a need for a more accurate and robust method and device for measuring filter usage in a filtered water container.